Characteristics of a speaker diaphragm greatly depend on the propagation velocity and the internal loss. Namely, a speaker diaphragm having the higher propagation velocity exhibits superior vibration characteristics in the high frequency sound range, and the one having the greater internal loss hardly causes the resonance phenomenon and reproduces the sounds of superior quality.
In general, however, diaphragms having the higher propagation velocity show the smaller internal loss while those having the greater internal loss are slow in the propagation velocity. Therefore, it has been important for a diaphragm to have an appropriate internal loss and an appropriate velocity of propagation, as well as smooth vibration characteristics.
Aiming to implement such a speaker diaphragm, it has been proposed, as shown in FIG. 27, to overlay paper sheet 72 on non-woven fabric mat 71 which is impregnated with phenol resin or the like thermosetting resin, and draw them in heat to be unitized by thermo compression bonding. Diaphragm 73 is completed by thermo-curing the thermosetting resin (Patent Document 1). Namely, in conventional speaker diaphragm 73, the hardness of the non-woven fabric which is a constituent of the diaphragm was raised by thermo-curing the thermosetting resin, and paper sheet 72 having a great internal loss was affixed unitized with non-woven fabric mat 71 in order to offer a speaker of quality sounds.
The above-described conventional diaphragm 73 showed smooth vibration characteristics for the whole sound range. However, the diaphragm 73 was unable to produce satisfactory vibration characteristics in the high frequency sound region because of the slow propagation velocity of paper sheet 72.
[Patent Document 1]                Unexamined Japanese Utility Model Publication No. S59-106289        